IS THE ORGANISM A MECHANISM? 647 



calories of heat are evolved ; and when carbon disulphide is 

 decomposed completely 28,700 gram-calories are evolved. Now, 

 if we calculate, from known data, the heats of combustion of the 

 carbon and hydrogen contained in a gram-molecular weight of 

 acetylene we shall find that it is only 256,900 gram-calories; 

 therefore the balance of 53,100 gram-calories must be absorbed 

 when carbon and hydrogen are synthesised as acetylene. Also 

 when carbon and sulphur are synthesised to form carbon 

 disulphide 28,700 gram-calories, the heat of dissociation of the 

 compound must also be absorbed. These reactions, the com- 

 bination of carbon and hydrogen to form acetylene, and carbon 

 and sulphur to form carbon disulphide, are therefore endothermic 

 reactions. In them the intrinsic energy of the final product is 

 greater than the intrinsic energy of the initial substances. 



The difference between the two kinds of chemical change — 

 exothermic and endothermic changes — is fundamental. Nearly 

 all substances which react with each other do so with the 

 evolution of heat. A few reactions occur in which heat is 

 neither evolved nor absorbed, but these are of an altogether 

 special kind. A few reactions also occur in which heat is 

 absorbed. These, also, are special chemical changes ; they are 

 not numerous ; and the products resulting from them are 

 unstable as a rule. 



Considering further the above reactions two things are to 

 be noted. First, exothermic reactions occur of themselves. 

 Immediately caustic soda is added to hydrochloric acid neutra- 

 lisation begins. Methane and oxygen do not react at ordinary 

 temperatures (or they react " infinitely slowly ") but an in- 

 finitesimal amount of energy starts the reaction which then 

 proceeds until it has been completed. Endothermic reactions, 

 on the contrary, do not occur of themselves : carbon and 

 hydrogen will not react by themselves to form acetylene, nor 

 will carbon and sulphur to form carbon disulphide. These re- 

 actions will not occur, as does the explosion of a methane-oxygen 

 mixture, when an infinitesimal " stimulus " is applied. In order 

 that they may take place a compensatory energy-transformation must 

 be set up, and in this compensating reaction an amount of energy 

 equal to that absorbed in the endothermic change is supplied. 

 I have emphasised the above sentence in order that the reader 

 may appreciate the importance that it has for our later dis- 

 cussion. 



